The present invention relates to a developing apparatus and a picture image forming apparatus; and, more particularly, the invention relates to a developing apparatus for use in an electrophotography apparatus, such as a copying machine, a printing machine and a facsimile apparatus. The present invention also relates to a picture image forming apparatus for forming a two-color image using the developing apparatus.
The general principle of operation of a picture image forming apparatus will be explained. First of all, an example of a tri-level two-color picture image forming apparatus will be explained with reference to FIG. 7 and FIG. 8.
The tri-level two-color picture image forming apparatus, or the tri-level xerographic apparatus for forming a two-color picture image, comprises mainly a charging means 11 and an exposing means 25. The charging means 11 applies a charge uniformly to a surface of a photoconductor 1, which charging means is arranged opposite to an outer peripheral surface of the photoconductor 1. The exposing means 25 generates an exposing light beam 14, which exposes one region with weak intensity or half power to form a positive latent image and another region with strong intensity or full power to form a negative latent image, in response to a picture image signal having a first color signal component and a second color signal component.
An electrostatic latent image having a tri-level distribution (see FIG. 8, wherein a region L1=Vo, a region L2=Vr, and the remaining region=Vw) is formed on the outer peripheral surface of the photoconductor 1 by the charging means 11 and the exposing means 25.
The tri-level two-color picture image forming apparatus comprises a developing apparatus 12 for forming the first color and a developing apparatus 13 for forming the second color. Both the developing apparatus 12 and the developing apparatus 13 employ a different charge polarity of toner with respect to each other. By using the developing apparatus 12, a high potential portion (Vo=the region L1) is developed through a normal development process where the toners are charged with a reverse polarity with respect to the charging polarity of the photoconductor 1.
Further, by using the developing apparatus 13, a low potential portion (Vr=the region L2) is developed through a reverse development process where the toners are charged with the same polarity as the charging polarity of the photoconductor 1. In this way, a two-color toner image is formed on the surface of the photoconductor 1. The recharging of the photoconductor 1 is performed through a pre-transfer charging means 24, so that the polarity of the toner image is made equal to either the positive polarity or the negative polarity.
The toner image now charged with one polarity is transferred onto a paper sheet 15 through a transferring means 23, and the toners are thermally fused on the paper sheet 15 through a fixing means 17, after which the toners are fixed on the paper sheet 15. Thus, with one rotation of the photoconductor 1, a printing process to produce a two-color picture image is carried out.
The above stated two-color developing method has been proposed in, for example, Japanese patent laid-open publication No. 48-57637 and U.S. Pat. No. 4,078,929. The use of a pre-transfer charging means 24 for recharging the toners to either the positive polarity or the negative polarity, for example, has been proposed in Japanese patent laid-open publication No. 48-53727.
Further, in the operation of the developing apparatus, various processes have been proposed. For example, FIG. 6 shows a conventional developing apparatus 2 where two-component developer mixed carriers with toners are used. In this developing apparatus 2, a developing roller 5 is arranged opposite to an outer peripheral surface of a photoconductor 1 with a developing gap provided therebetween. The developing roller 5 is constituted by a columnar form magnet body 4 which is arranged inside of a rotating developing sleeve 3.
Plural magnetic poles S, N, S, N, . . . are magnetized at an outer peripheral portion of the magnet body 4, and a developer 7, formed of a mixture of toners with carriers, is attracted according to the magnetic force of the magnet body 4, so that a magnetic brush is formed by the developer 7 held on an outer peripheral surface of the developing sleeve 3. By causing the developer 7 to contact the surface of the photoconductor 1, an electrostatic latent image on the surface of the photoconductor 1 is developed, whereby a toner image is formed.
The developer 7 is transported in the direction of the arrow, with the counter-clockwise rotation of the developing sleeve 3 to a developing zone, while the amount of the developer 7 carried on the developing sleeve 3 is regulated by passing the developer 7 through a regulating gap formed between a regulating plate 6 and the developing sleeve 3.
In the developing zone arranged between the developing roller 5 and the photoconductor 1, the magnetic brush is formed along the magnetic lines of force formed through the magnetized pole plates 8 having, for example, the same polarity, so as to constitute adjacent double magnetic pole units having the same polarity.
In the developing zone, as already indicated, the magnetic brush is formed along the magnetic lines of force produced by adjacent magnetic poles having the same polarity. With respect to this magnetic brush, in the center portion of the double magnetic poles having the same polarity, the developer 7 can easily move to the surface of the photoconductor 1 because the constriction for the developer 7 produced by the magnetic lines of force becomes weak. Accordingly, it is possible to increase the developing density.
The above described developing unit which uses the same polarity method, for example, has been proposed in Japanese patent laid-open publication No. 55-101969, Japanese patent laid-open publication No. 3-291680, and Japanese patent laid-open publication No. 4-338781. Further, a method for applying a bias voltage comprised of an AC bias voltage and a DC bias voltage has been proposed in, for example, Japanese patent laid-open publication No. 60-168177, and Japanese patent laid-open publication No. 3-109582.
In the above stated conventional tri-level picture image forming apparatus, as shown in FIG. 8, the charging potential Vo is divided into two levels relative to the intermediate potential Vw, so that a first color electrostatic latent image and then a second color electrostatic latent image are formed with similar potential differences at an upper portion and a lower portion.
Accordingly, the potential difference between the developing bias potentials Vb1 and Vb2 of each of the first color and the second color and the latent image potential, namely, the first developing potential difference .DELTA.V1 and the second developing potential difference .DELTA.V2, becomes small. As a result, it has been difficult to obtain a high picture image density.
Further, in a pre-stage of the first color developing apparatus 12, the first color electrostatic latent image and the second color electrostatic latent image are formed at the same time by the exposing light beam 14. Accordingly, in the first color developing apparatus 12, steps must be taken when developing the first color electrostatic latent image to not disturb the second color electrostatic latent image.
However, in the second color developing apparatus 13, during the second color developing process, there is a tendency to disturb the first color toner image. The first color image has been formed on the outer peripheral surface of the photoconductor 1 when the magnetic brush brushes the photoconductor 1 with the developer 7, which is held on the developing sleeve 3.
Further, in the conventional developing apparatus 2 employing the same polarity system, there are no proposals for obtaining preferable conditions with respect to the constituting condition between the developer 7 and the magnetic pole units having the same polarity, with respect to the brushing force against the photoconductor 1 during the developing process, and with respect to the developing characteristics.